1. Field of the invention
The present invention relates to a diffractive optical element and scanning optical apparatus using the same and, more particularly, to an apparatus which records image information by causing a deflection element to deflect a light beam emitted by a light source means formed from a semiconductor laser and optically scanning a surface to be scanned through a scanning optical element (imaging element) having f-.theta. characteristics, and is suitable for an image forming apparatus such as a laser beam printer (LBP) or digital copying machine having an electrophotography process.
2. Related Background Art
In a conventional scanning optical apparatus used in a laser beam printer, digital copying machine, or the like, a light beam which is optically modulated in accordance with an image signal and is output from a light source means is periodically deflected by an optical deflector such as a rotary polyhedral mirror (polygon mirror), and is focused to form a beam spot on the surface of a photosensitive recording medium (photosensitive drum) by a scanning optical element (imaging element) having f-.theta. characteristics. Then, the beam spot is scanned on that surface to record an image.
FIG. 1 is a schematic sectional view showing principal part of a conventional scanning optical apparatus of this type.
Referring to FIG. 1, a divergent light beam emitted by a light source means 91 is converted into a nearly collimated light beam by a collimator lens 92, and the light beam (light amount) is limited by a stop 93. Then, the light beam enters a cylinder lens (cylindrical lens) 94 having a predetermined power in only the sub-scanning direction perpendicular to the drawing surface. Of the nearly collimated light beam that enters the cylinder lens 94, light components in the main scanning section directly emerge as a nearly collimated light beam. In the sub-scanning section perpendicular to the drawing surface, light components are focused to form a nearly linear image on a deflection surface (reflection surface) 95a of an optical deflector 95 that comprises a rotary polyhedral mirror (polygon mirror).
The light beam deflected and reflected by the deflection surface 95a of the optical deflector 95 is guided onto a photosensitive drum surface 98 as a surface to be scanned via a scanning optical element (f-.theta. lens) 96 having f-.theta. characteristics. By rotating the optical deflector 95 in the direction of an arrow A, the light beam scans the photosensitive drum surface 98 in the direction of an arrow B. In this way, an image is recorded on the photosensitive drum surface 98 as a recording medium.
Conventionally, various scanning optical apparatuses using plastic lenses have been proposed as scanning optical systems because of their capability of highly accurate aberration correction using aspherical surfaces and cost reduction by injection molding.
However, a plastic lens largely changes in its aberration (especially errors in focus or magnification) due to environmental variations. This poses a serious problem in a scanning optical apparatus having a small spot diameter.
Recently, to compensate for aberration variations unique to a plastic lens, some apparatuses have been equipped with diffractive optical elements as a scanning optical system, as proposed in, e.g., Japanese Patent Application Laid-Open No. 10-68903. In this prior art, for example, when ambient temperature increases, chromatic aberration is generated using a diffractive optical element in advance so as to compensate for a change in aberration due to a decrease in refractive index of a plastic lens with a change in aberration due to wavelength variation of a semiconductor laser as a light source. When a diffractive optical element is used singly, the element has a predetermined thickness. Hence, the element manufactured by injection molding is excellent in molding properties.
A diffractive optical element is very useful as the optical system of a scanning optical apparatus. However, the light utilization efficiency (to be referred to as a diffraction efficiency .eta. hereinafter) of a diffractive optical element changes depending on conditions, unlike a refractive optical element. This will be described below using a diffraction grating model.
FIG. 2 is an explanatory view of a diffraction grating-model. This diffraction grating model comprises a continuous grating having a grating pitch of P .mu.m and a grating depth of h .mu.m. The ratio of the grating pitch to the grating depth is called an aspect ratio AR, and it is defined that AR=grating pitch P/grating depth h. A light beam incident on the substrate of the diffraction grating model at an angle of incidence .theta.i is diffracted and emerges in the direction of designed diffraction order.
FIG. 3 is an explanatory view showing the dependence of the diffraction efficiency on the angle of incidence when the aspect ratio AR is 4 in the above diffraction grating model. As is apparent from FIG. 3, the diffraction efficiency greatly changes depending on the angle of incidence, and especially, the diffraction efficiency of a light beam incident at a large angle of incidence lowers.
FIG. 4 is an explanatory view showing the dependence of the diffraction efficiency on the aspect ratio when the angle of incidence on the grating portion is .theta.i=0 in the above diffraction grating model. In FIG. 4, the grating depth h is not changed while the aspect ratio AR is changed by changing the grating pitch P. When the aspect ratio is lower than 4, the diffraction efficiency abruptly decreases.
As is apparent from the above two conditions, when a diffractive optical element is used as the scanning optical system of a scanning optical apparatus, the diffraction efficiency lowers in the off-axis region where the angle of incidence is large and the aspect ratio is low, so the uniformity of an image plane illuminance on a surface to be scanned degrades.